In recent years, micro-electro-mechanical systems (MEMSs) have been attracting much attention. These are intelligent systems, of sizes ranging from several microns to several hundreds of microns, and consisting of a plurality of components such as sensors, actuators, and electronic circuits fabricated as single units by means of IC manufacturing technology. However, the study of micro-actuators is still at an initial stage. Among various types of micro-actuators, ultrasonic motors using a piezoelectric material such as PZT are currently a subject of particular interest.
As described in Published Unexamined Patent Application (PUPA) No. 63-220773, in conventional ultrasonic motors, a progressive wave is generated in an elastic material by causing two piezoelectric ceramics that are attached to the elastic material to expand and contract repeatedly. This is achieved by applying voltages of different phases to the piezoelectric ceramics. An object placed on the elastic material moves in a direction opposite to that of the progressive wave.
However, these conventional ultrasonic motors have several problems. First, since surface waves are generated over the entire motor area, it is difficult to control position locally. That is, when two objects are placed on an actuator, it is difficult to control the respective positions of the two objects individually. Furthermore, since the position of an object can be obtained only as an integral of its velocity, fine positioning cannot be attained without using a closed-loop control system.
T. Furuhata, T. Hirano, and H. Fujita, in "Array-Driven Ultrasonic Micro-actuators," International Conference Solid-state Sensors and Actuators, 1991, pp. 1056-1059, disclose a micro-positioning device comprising a substrate, a plurality of micro-actuators arranged on the substrate in an array, and a moving member placed on the micro-actuators. Fine positioning is done in a predetermined direction such that the moving member is friction-driven by the rotational motion of the ends of the micro-actuators. Each micro-actuator consists of a driving section for applying a driving force so as to generate vertical motion causing displacement and a mechanism for converting the vertical motion into rotational motion causing displacement in the horizontal direction. This micro-positioning device can solve the problems of the conventional ultrasonic motors mentioned above.
However, the device described in the above paper, in which the moving member is moved horizontally by first generating vertical motion and then converting it into rotational motion, needs some improvements with respect to the speed of the moving member.
Furthermore, when the driving force is generated by using piezoelectric materials in the above device, the piezoelectric materials need to be displaced in the vertical direction. In MEMS, to save space and facilitate integration with electronic circuits, it is desirable that a single-layer piezoelectric film be used, rather than the conventional multi-layer piezoelectric film. However, it is difficult for a single-layer piezoelectric film to provide a large vertical displacement. Therefore, a driving technique suitable for a single-layer piezoelectric film is now required.